The Fan speed depeding on your model is based upon A/C system pressure. Some models have a high pressure switch which turns the fans to High speed when the pressure reaches 16 Bar. The other type of sensor transmits a signal to the ECU which then decides how best to manage things. FYI the a/c system high pressure cut-out pressure is 32bar, with a reset point of 24 bar.

If you have a high side pressure of 100psi and low side of 80 with the compressor running something is wrong. Maybe an issue with the expansion valve, or the compressor itself. Really you need put a set of gauges on it with the a/c system off then turn it on and watch the gauges and see what they show. Normal High Side should be in the region of 16bar with the low side around 1.5-2.5 bar. (dependant on ambient temp)